1998 Lexus LS 400FACTORY OEM SERVICE PROCEDURE

Fuel Pump Control Circuit: Es300

  1. Check fuel pump operation. See FUEL PUMP OPERATION . If fuel pump does not operate, go to next step. If fuel pump operates, check starter signal circuit. See STARTER SIGNAL CIRCUIT  under COMPUTERIZED ENGINE CONTROLS. Repair as necessary.
  2. Check ECM power source circuit. See ECM POWER SOURCE CIRCUIT (IGNITION KEYED)  under COMPUTERIZED ENGINE CONTROLS. Repair as necessary. If circuit is okay, go to next step.
  3. Check circuit opening relay. See RELAYS  under MODULES, MOTORS, RELAYS & SOLENOIDS. Replace circuit opening relay as necessary. If relay is okay, access ECM connectors behind glove compartment. Turn ignition on. Backprobing connector, measure voltage between chassis ground and terminal No. 18 (Green/Red wire) at ECM E9 connector. See Figure-10.
  4. If battery voltage is not present, check for an open circuit in wiring harness between EFI main relay and circuit opening relay or between opening relay and ECM. See appropriate wiring diagram in WIRING DIAGRAMS article. Replace as necessary. If battery voltage is present, go to next step.
  5. Check fuel pump. See FUEL PUMP RESISTANCE . Replace fuel pump as necessary. If fuel pump is okay, check for an open circuit in wiring harness between circuit opening relay and fuel pump or between fuel pump and ground. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair as necessary. If wiring is okay, replace ECM and retest.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.

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